1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a facsimile machine, a printer, etc., including a detection mechanism for precisely measuring the speed of a rotary member such as a transfer belt.
2. Description of the Background
A color copier or printer generally includes a plurality of developing devices surrounding a photo conductor (i.e., a single drum type apparatus) or a single developing device around a single photoconductor (i.e., a tandem type apparatus). The single drum type apparatus is advantageous because it is smaller, thereby reducing the overall cost of the apparatus. On the other hand, the tandem type apparatus is advantageous because the printing speed is faster.
Accordingly, the tandem type apparatus has recently been used especially because the printer market is demanding the printing speed of color copiers be the same as monochromatic copiers.
In addition, the tandem type apparatus includes two types. A first type is called a direct transfer type in which a toner on a photoconductor is transferred to a sheet conveyed by a transfer belt. The second type is called an indirect transfer type in which a toner on a transfer belt is transferred to a sheet by a second transfer device. However, both types of devices have a same problem in properly overlaying images to provide a color copy.
Therefore, an image forming apparatus must precisely determine and control the speed of the rotary member such as a transfer belt, a conveying belt, etc., to precisely overlay plural images. For example, Japanese Patent Laid-Open No. 11-024507 (JP '507) discloses a device that determines a speed of the rotary member by detecting a scale on the rotary member. However, in this publication, the location of the scale is in a position in which image transfer occurs. Therefore, the scale negatively affects the image transfer. JP '507 is also incorporated in its entirety.